1. Field of the Invention
The present invention relates to a liquid crystal display panel which is equipped to a word processor, personal computer or other electronic devices, and a method for manufacturing a light reflecting film of the liquid crystal display panel.
More particularly, the invention concerns a liquid crystal display panel which is of a reflection type wherein display is made by reflecting light having entered from the outside without using a back-light and which is small in power consumption, and a method for manufacturing a light reflecting film of the liquid crystal display panel.
2. Description of the Related Art
As a liquid crystal display panel there are a twisted nematic type which darkens due to the presence of two polarizing plates although it can have a high contrast, a super twisted nematic type which can have a high contrast and be driven in a simple way, a phase transition type guest/host type which is high in luminance and less in parallax error because of no need to use a polarizing plate, etc.
In the above-mentioned phase transition type guest/host liquid crystal, dichroic pigment is added to and mixed with the guest molecules in the host liquid crystal and, by varying the arrangement of the liquid crystal molecules by means of the voltage applied to the liquid crystal, the light absorptance of the liquid crystal layer is varied.
FIG. 5 is a typical view of a conventional example, illustrating the section of one pixel of a liquid crystal display panel using a phase transition type guest/host liquid crystal.
In FIG. 5, between an active matrix substrate 51 and an opposing substrate 52, there are formed sequentially from an upper surface of the active matrix substrate 51 thin film transistors 53, interlayer insulating film 54, pixel electrodes 55, light reflecting films 56 formed on the surface of the pixel electrodes, orientation film 57, phase transition type guest/host liquid crystal 58, orientation film 59, opposing electrode 60, and color filter 61 in this order.
Since FIG. 5 is intended to illustrate one pixel, the whole of the liquid crystal display panel is not illustrated therein. However, on the upper surface of the active matrix substrate 51, the thin film transistors 53 corresponding to a numeric value three times as large as the number of the pixels are formed in the form of a matrix. On the upper surface of the active matrix substrate 51, the pixel electrodes 55 corresponding to a numeric value three times as large as the number of the pixels are formed by being insulated by the interlayer insulating film 54.
In the above-mentioned liquid crystal display panel, incident light 62 having entered from the outside transmits through the opposing substrate 52, color filter 61, opposing electrode 60, orientation film 59, phase transition type guest/host liquid crystal 58 and orientation film 57 and then is reflected by the light reflecting film 56, whereupon the resulting light advances through a path opposite to that through which the incident light entered and goes outside, thus becoming a reflected light 63.
Also, the pixel electrode 55 is made of aluminum and is provided with fine concavities and convexities in order to scatter and reflect the light having entered thereinto by its surface. And, the concavities and convexities of the pixel electrode 55 are so designed that the light having entered from a direction inclined by an angle of 30xc2x0 with respect to the direction orthogonal to the pixel electrode 55 may be strongly reflected in this direction orthogonal thereto.
And, the surface of the pixel electrode 55 is chemically formed with the concavities and convexities by etching which uses acid, such as hydrofluoric acid treatment, thereby constituting the light reflecting film 56.
Between the opposing electrode 60 and the pixel electrode 55 there is sealed the phase transition type guest/host liquid crystal 58. In its natural state, the phase transition type guest/host liquid crystal 58 is such that the host liquid crystal molecules are helically arranged. In a state where a voltage is applied thereto, the helicoid is released with the result that the molecules are arranged vertically with respect to the opposing substrate 52. For this reason, the phase transition type guest/host liquid crystal 58 makes it possible to obtain a high contrast without using a polarizing plate.
However, whereas the phase transition type guest/host liquid crystal enables the procurement of a high contrast with no use of a polarizing plate, when driving it the application of a high voltage was needed for releasing the helicoid.
Accordingly, because of its large power consumption, the phase transition type guest/host liquid crystal had the problem that when it was applied to portable electronic devices, the time period in which a battery was used therein was short.
Also, in the pixel electrode of the phase transition type guest/host liquid crystal, time and labour were needed to perform the treatment for producing the light reflecting film whose concavities and convexities causing uniform scatter of light have been formed by etching.
Also, in the fabrication of the light reflecting film having concavities and convexities by etching, there was a limitation with respect to enlarging the depth of the concavities and convexities and so the scattering of light was not sufficient, the resulting structure therefore having a problem with the brightness demanded of the liquid crystal display panel.
Further, while electronic devices are for the most part arranged to reflect light, which is incident thereupon from a direction inclined by an angle of 30xc2x0 with respect to the direction orthogonal thereto, in this direction orthogonal thereto, there are also electronic devices such as televisions which are wanted to be viewed by a plurality of persons or electronic devices such as wrist watches in which it is better to reflect the incident light from the orthogonal direction in this orthogonal direction. With regard to the light reflecting film of the liquid crystal display panel used in the electronic devices, it was difficult to control the configuration of the concavities and convexities so as to enable the uniform reflection of light whose incident angle was different.
The present invention has been made in order to solve the above-described problems and an object of the present invention is to provide a liquid crystal display panel whose image plane is high in luminance, by making up the light reflecting film into a porous layer and thereby making the light to be scattered large in amount.
Another object of the present invention is to provide a method for manufacturing the light reflecting film of the liquid crystal display panel, which enables the simple control of the configuration of the porous layer by subjecting the light reflecting film to anodic oxidation.
To attain the above objects, according to a first aspect of the present invention, the liquid crystal display panel according thereto uses a phase transition type guest/host liquid crystal shown in FIG. 1. It comprises pixel electrodes 15 disposed on an active matrix substrate 11 in the form of a matrix with an interlayer insulating film 14 therebetween and respectively connected to thin film transistors 13 and light reflecting films 16 formed in an upper surface of the pixel electrodes, the light reflecting films consisting of a porous layer.
According to a second aspect of the present invention, the light reflecting film of the liquid crystal display panel of the present invention is arranged such that the porous layer of the same configuration is multilayered.
According to a third aspect of the present invention, the light reflecting film of the liquid crystal display panel of the present invention is arranged such that the porous layer of a different configuration is multilayered.
According to a fourth aspect of the present invention, the liquid crystal display panel of the present invention is composed of switching thin film transistors 13 respectively connected to the pixel electrodes disposed in the form of a matrix and driving thin film transistors disposed around a display portion.
According to a fifth aspect of the present invention, each of the pixel electrodes and the light reflecting films of the present invention is made of material having at least aluminum as its main component.
According to a sixth aspect of the present invention, each of the pixel electrodes and the light reflecting films of the present invention is made of material containing scandium, having at least aluminum as its main component.
According to a seventh aspect of the present invention, the light reflecting film of the liquid crystal display panel of the present invention consists of an anodic oxide film.
According to an eighth aspect of the present invention, a method for manufacturing the light reflecting film of the liquid crystal display panel of the present invention comprises the step of forming by anodic oxidation the light reflecting film consisting of a porous layer with respect to an upper surface of the pixel electrode made of material having at least aluminum as its main component.
According to a ninth aspect of the present invention, a method for manufacturing a light reflecting film of the liquid crystal display panel of the present invention controls the configuration of the porous layer by changing at least one of the concentration of aqueous solution of oxalic acid, the voltage value, the current value, the time duration in which the voltage is applied, and the time duration in which the current is applied.